1. Field of the Invention
The present invention relates to a switching system for an electronic switcher which system employs common channel signalling and channel-associated signalling. More particularly, the invention relates to a method and a system which are capable of employing channel-associated signalling for connecting a fundamental call if failure results in disabling a common channel signal link for transferring control information for the call processing.
2. Description of the Prior Art
The relation between a channel-associated signalling and a common channel signalling will be explained by using FIG. 10 and FIG. 11. FIG. 10 shows the aspects of channel-associated signalling. In channel-associated signalling, the transmission of the information necessary for controlling the connection between exchange offices is performed through respective speech paths. The information necessary for controlling the connection between the exchange offices is transmitted and received through the speech path by means of DP (dial pulse) signal and MF (multi-frequency) signal.
FIG. 11 shows the aspects of common channel signalling. In common channel signalling, the information necessary for controlling the connection between the exchange offices is transmitted and received through a separate leased signal path different from the speech path. This signal path is commonly used by a number of speech paths. Further, this signal path, that is independently established for transmitting the information necessary for controlling the connection between the exchange offices, is referenced as a "common channel signal link."
Traditionally, the following measures are provided for failure caused in the common channel signalling to transfer the control information for the call processing.
As described in the Official Gazette JP-A-62-48175, one of those measures serves to switch the common channel signalling to the channel associated signalling when it is determined that some failure results in disabling the signal link.
The foregoing prior art, however, does not obviously propose a concrete means for carrying out the changeover control of both signalling modes. In particular, nothing is disclosed as to how to control a call being active at a time point or later when these signalling systems are switched to each other.
Heretofore, the following two methods were relied upon as back-up means when the common channel signal link fails.
The first method will be explained by using FIG. 12. According to such first method, a common channel signal link for back-up is provided. When a failure occurs in one of the common channel signal links, the failed common channel signal link is switched to another common channel signal link. Consequently, the information necessary for controlling the connection between the exchange offices can be continuously transmitted.
The switch to the back-up common channel signal link at the time of failure and the change back at the time of failure recovery depends on the change-over and change back processes defined by IEE Report by Welch, Institution of Electrical Engineers Report, "Signalling in Telecommunications Network." pp. 260-321, or CCITT No. 7 signal system shown in Telephony, "Common Channel Signalling based on CCITT No. 7," pp 85-98 (Feb. 9, 1981).
The second method will be explained by using FIG. 13. According to such second method, there is provided a reroute, from a route to an opposite office comprising a speech path controlled by the common channel signal link, to another route at the time of failure in the common channel signal link.
In FIG. 13, a reroute from the route 1, controlled by the common channel signalling to the route 2, controlled by the channel-associated signalling is provided. In both methods, problems existed in connection with the effective use of trunk lines between exchange offices.